1. Field of the Invention
The present invention relates to a circuit module, and specifically, relates to a circuit module including an isolator.
2. Description of the Related Art
As an existing circuit module, for example, there is known a transmission device disclosed in Japanese Unexamined Patent Application Publication No. 2008-154201. The transmission device includes a power amplifier, an isolator, a switch, a first duplexer, and a second duplexer. The power amplifier is a so-called multi-band power amplifier, which amplifies and outputs a transmission signal of a first frequency band and a transmission signal of a second frequency band, the first frequency band and the second frequency band being different from each other. The isolator allows the transmission signal of the first frequency band and the transmission signal of the second frequency band, which are outputted from the power amplifier, to pass therethrough, and does not output, to the power amplifier, a signal inputted from the opposite side of the power amplifier. The switch outputs the transmission signals having passed through the isolator, to the first duplexer or the second duplexer. The first duplexer is a filter that allows the transmission signal of the first frequency band to pass therethrough, and the second duplexer is a filter that allows the transmission signal of the second frequency band to pass therethrough.
In the transmission device as described above, impedance matching between the output impedance of the isolator and the input impedance of the first duplexer and the input impedance of the second duplexer needs to be optimized for the transmission signal of the first frequency band and the transmission signal of the second frequency band, respectively. Although not described in Japanese Unexamined Patent Application Publication No. 2008-154201, a matching circuit for providing impedance matching is generally provided in the isolator.
Since the frequency bands of the signals that pass through the first duplexer and the second duplexer are different from each other, each duplexer is designed such that the input impedance is 50Ω in the passband of each duplexer. Thus, it is difficult for a matching circuit on the output side of the isolator to provide impedance matching to both the input impedance of the first duplexer and the input impedance of the second duplexer. Therefore, the output-side matching circuit of the existing isolator is designed so as to provide matching in a frequency that is optimal for both the passing frequency band of the first duplexer and the passing frequency band of the second duplexer, for example, in an intermediate frequency between the first frequency band and the second frequency band. However, with such designing, the impedances do not match for both the signal of the first frequency band and the signal of the second frequency band, and thus a loss occurs in the transmission signal. Further, in order to prevent the occurrence of a loss in the transmission signal, a structure is considered in which matching circuits are provided between the switch and the first duplexer and between the switch and the second duplexer. In this case, however, since a plurality of matching circuits is used, the number of elements is increased and the size of the transmission device is increased.
As described above, the transmission device described in Japanese Unexamined Patent Application Publication No. 2008-154201 causes a loss in the transmission signal and needs to be provided with matching circuits at three locations. Thus, the number of elements is increased, and the size of the transmission device is increased.